The invention relates generally to methods for monitoring and controlling processes used in forming features on semiconductor substrates. More specifically, the invention relates to a method for detecting an endpoint in semiconductor substrate processing.
In semiconductor manufacturing, various combinations of processes such as etching, thin-film deposition, and chemical-mechanical polishing are used to form features on a semiconductor substrate. The features are formed by selectively removing materials from and selectively depositing materials on the surface of the semiconductor substrate. While forming the features, the semiconductor substrate is monitored to determine when an endpoint has been reached in the process. An endpoint could be a point at which the process conditions should be changed or a point at which the process should be stopped.
Deep trench and recess etch processes are used in fabrication of semiconductor devices such as dynamic random access memory (DRAM) and embedded DRAM (eDRAM). A DRAM (or eDRAM) cell contains transistors and capacitors for storing information. Typically, the storage capacitors are installed in trenches in a semiconductor substrate. A typical process for forming a trench capacitor involves etching a deep trench in a semiconductor substrate, filling the trench with polysilicon, and etching down the polysilicon to form a recess in the trench. Other materials, such as a dielectric material, may also be deposited in the trench or recess and etched as necessary to form a desired storage structure. Typically, the trench has a high aspect ratio (i.e., greater than 1.0, where “aspect ratio” is defined as height/width). In the current technology, for example, the depth of the trench is typically several microns deep, while the width of the trench is typically on the order of 300 nm. As advances are made in integration technology, the width of the trench is expected to get even smaller, e.g., shrink down to 90 to 100 nm.
FIG. 1A shows a typical semiconductor substrate 100 having a substrate layer 102, typically made of silicon, a pad layer 104, typically made of silicon dioxide, and a mask layer 106, typically made of silicon nitride. A thin film of photoresist mask 108 may also be deposited on the mask layer 106. Prior to forming a deep trench in the substrate 100, an area 110 of the photoresist mask 108 where the trench will be formed is removed, causing the underlying layer, i.e., the mask layer 106, to become exposed. The substrate 100 is then placed in a process chamber (not shown), such as a plasma chamber, and the trench is etched through the mask layer 106 and pad layer 104 into the substrate layer 102. FIG. 1B shows a trench 112 etched in the substrate 100. After etching the trench 112 in the substrate 100, the remaining photoresist mask (108 in FIG. 1A) is removed.
FIG. 1C shows the trench 112 in the substrate 100 backfilled with polysilicon 114. During the backfill process, a blanket of polysilicon 116 is formed over the mask layer 106. Typically, a small dish (or depression) 118 appears above the opening of the trench 112 as a consequence of the backfill process. Before forming a recess in the polysilicon 114 in the trench 112, all or a portion of the blanket of polysilicon 116 is removed by a planarization process, such as planar layer etching or chemical-mechanical polishing. FIG. 1D shows the substrate 100 after the planarization process. A depression 120 may appear above the opening of the trench 112 as a consequence of the planarization process. After the planarization step, the polysilicon column 114 in the trench 112 is etched down to a predetermined depth to form a recess. FIG. 1E shows a recess 122 formed above the polysilicon column 114.
The depth of the recess 122 relative to a reference point in the substrate 100, e.g., the bottom of the sacrificial mask layer 106, is usually a critical dimension. However, various factors make it challenging to accurately form a recess having a desired depth. One factor is that the opening of the trench through which the recess is etched is very tiny, e.g., on the order of 300 nm or less. Thus, the etch process must be carefully controlled to ensure that the etching is confined to the trench. Another factor is that the depression above the polysilicon column can easily be on the same order as the accuracy or even the absolute depth of the recess to be etched. Thus, the dimensional control limits are very tight. Another factor is that there are incoming material variations from one substrate to another, e.g., variations in thickness of the mask layer (e.g., as a result of the planarization process) and the depth of the depression above the polysilicon column. Without knowledge of these variations, it would be difficult to determine how far down to etch the polysilicon to make the required recess depth.
In order to accurately form a recess of a desired depth, it is important to have an accurate and reliable method of detecting an endpoint in the etching process. Optical diagnostic methods are typically used to detect endpoints in patterned substrate processing because they are non-intrusive. Optical emission spectroscopy is the most widely used optical diagnostic method for detecting an endpoint. The method involves monitoring plasma emissions for a change in the species of the plasma, where a change occurs when moving from one layer of the substrate to another layer. The response of this method is typically delayed because it monitors the plasma state instead of the substrate state. Optical emission spectroscopy is generally unsuitable for deep trench and recess etching as well as other etch applications where there is no effective etch stop layer.
Single-wavelength interferometry is another example of an optical diagnostic method that is used to detect an endpoint. The interferometry approach involves directing a light beam on the substrate surface. The reflected signals from the substrate combine constructively or destructively to produce a periodic interference fringe as a film, trench or recess is being etched. The phase of the interference fringe depends on the path length of the light beam through the thickness of the layer being etched. During etching, the observed number of periods of a measured interference fringe is correlated with a calculated reduction in the thickness of the layer or the change in the depth of the trench or recess being etched to estimate an endpoint in the process. The interferometric endpoint detection method involves counting the number of fringes evolved during the etch. When a predetermined number of fringes corresponding to the thickness of material to be removed has been counted, the etching process is stopped.
Single-wavelength interferometric approaches are limited in their ability to monitor etching applications such as recess etching. One reason for this is that they monitor relative changes in vertical dimensions of structures on the substrate as opposed to absolute vertical dimensions of structures. Thus, they cannot compensate for incoming material variations from one substrate to another, such as variation in thickness of mask layer, variation in starting depth of trenches, variation in pattern densities, and variation in wafer orientation. As previously mentioned, without knowing these incoming material variations, it would be difficult to accurately determine how much material to remove via etching. Another reason is that as the structures get smaller (e.g., smaller than the wavelength of the incident light) and deeper the contrast of the fringes evolved from the substrate drops and any small noise can wash out the fringes, making it impossible to determine when an endpoint has been reached in the process.
Spectroscopic ellipsometry, polarimetry, and reflectometry are examples of optical diagnostic methods that can be used in conjunction with rigorous optical modeling techniques to determine the absolute vertical and lateral dimensions of features of special test structures such as one-dimensional gratings on a patterned substrate. However, these techniques are limited to inline metrology applications (i.e., pre- and post-processing metrology) rather than in situ diagnostics since they involve measurements only on special test structures and also a significant computational load. Efforts have been made to combine the use of spectroscopic ellipsometry and simple, considerably less accurate, modeling techniques for in situ diagnostics.
From the foregoing, there is desired a robust, easy-to-use, and accurate method for in situ diagnostics that will facilitate detecting an endpoint in substrate processing even when the structures of interest are much smaller than the wavelength of the incident light.